Electron optical system



Dec. 31, 1940. sc s I 2,227,033

ELECTRON OPTICAL SYSTEM Filed March 30, 1938 Fig.7-

Jnveniop Patented Dec. 31, 1940 UNITED ST T P 1511\11 OFFIC ELECTRON OPTICAL SYSTEM. i Kurt Schlesinger, Berlin, Germamnjassignor, by

mesne assignments, t -poration of New Yor Loewe Radio, Inc-, a cor- ApplicationMarch 30, 1938, Serial No.1 98,829

. In Germany April 4, 1332; 5 Claim s. (01. 256- 27) I This application is a. continuation-in-part of my copending application Serial No. 663,593 filed.-

in the United States Patent Office on March 30, 1933. The present invention relates to cathode 5 ray tubes as used for television and similar purposes in which the image point is moved over the screen and at the same time varied in intensity.

An object of the invention is a cathode-ray .tube which enables'to produceimage points or.

'spots, the size and form. of which are constant and are idependent of their intensity.

A further object of the invention is to pro duce image points or spots of an exactly predetermined form and size. 7 a

. A still further object is to prodoce image points or. spots which are independent of the distribution of the emission on thecathode and in consequence also of the length of time for which the cathode has been in operation, so that the i i constancy of the image points or spots is maintained during the entire life of the tube.

It is an essential object oftheinvention that a preliminarily concentrated bundle of electron rays governed by space charge control is so projected by a subsequent or after-connected elec-. tron-optical system that the focus point of this bundle falls in the plane of the imagescreen.

Still further objects of the invention 'will be seen in the following description.

The invention may be moreIfully understood from the appended drawing, whereof Fig. 1 shows a part of a tubeaccording to the invention and Fig. 2 a modification of 'Fi 1.

In the drawing, H is thehqiiipotentialcath ode indirectly heated by the resistance heater l2 and provided at its right end'with an electron,

emissive spot or coating. The; usual Wehnelt 40 cylinder l3 has anegative potential with respect I to the cathode as indicated by a connection to the potentiometer 23. Between a target or screen I1 (preferably a fluorescent screen) and the cathode are arranged three apertured electrodes I4, I 5 and I6. These electrodes are longitudi- 'nally spaced and axially aligned with the cath- I ode II and Wehnelt cylinder [3, and the electrode l4 may be referred to as an input anode and the electrode l5 as an electrode interme 5Q diate the other apertured-electrodes I4 and I6.

(The deflecting plates and any additional electrodes which the tube may c ontain are not shown.) The electrode or input anode i4 is.

connected to a moderate positive potential, say

of the full potential of the sec nd. anode l6,-

whilst the apertured electrode i5 is provided with a potential which, dependingon the distances, forms andpotentials of the other electrodes, may beslightly negative or positive relatively to=the cathode ll. In the examples illus- 5 trated. the electrode I5 is shown slightly positivewith respect to the cathode, it being obvious from the figures that the potentials applied to the various electrodes may be varied by shifting the leads along the element 23 as in- 10 dicatedby the arrows on each of these leads.

By. the action of the Wehnelt cylinder the cathode ray bundle (dotted. lines I9) is concentrated upon the small aperture I8 of the aperturecl electrode orinput anode M so that the 15 rays have a crossing point at or in the vicinity of this aperture. According to the invention the subsequent or second electron-optical system focusses the bundle IS on the screen l1 so that the bundle has two crossing points or points of 20 I convergence.

It; is essential'that the potential of the bundle entering the second electron-optical device be constant; Preferably. the arrangement is so dimensioned, thatth diameterof the bundle 5 I ofrays falling on the aperture I8 is not greater than the (preferably very small) diameter of that aperture.

As explained above the tube is based on the conception that the second electron-optical sys- 3Q tem under all circumstances does not reproduce on the image screen the cathode itself, but

the aperture of the input anode l4. Careful investigations have shown that this assumption is certainly correct, but only if applied vto-tubes 35 whichoperatewith relatively low strengths of ray current, and that when employing tubes with greater strengths of raycurrent (approximately-more than milliampere, and in particular more than .milliampere) entirely different 40 conditions prevail. v ,It was ascertained that the system in the case of tubes of this nature behaves in the same manner as if the radiating surface (i. .e., the surface acting virtually as electron-emitting means) 5 ness of the image point or spot is no longer ensured to full extent.

Accordingly the system illustrated in Fig. 1 is only able to. supply an image point which is equally sharp in the case of every strength of light when the distance of the object point (i. e., the point to be reproduced) from the system is the same in the case of each intensity. As object point there may be regarded the point of intersection of the trajectories of the individual electrons comprising the cathode ray bundle between the cathode and the input anode Hi. The position of this intersection point varies with changes in the intensity of the cathode rays. With the usual form of cathode, the main emission surface or level surface will in thecase of strong or bright? rays extend along the surface of the cathode, i. e., such level surface will coincide with the surface of the cathode, and the level surface in this position assumes the form of the cathode surface, but in the case of weak or dark rays the said level surface will differ greatly from the cathode surface; Thus, a variation takes place in the form of the level surface upon change over from-'thedark to the light condition, and accordingly also a shifting of the position of the object point which, as set forth above, is dependent on the form of the level" surface. In consequence the tube, generally speaking, is unable to supply the image points or spot with the same sharpness in the case of every intensity.- Although the variation in the diameter of the image" point is relatively small upon changeover from dark to light, the same is nevertheless found to be disturbin particularlyif the tube is employed for television purposes. H i

The cathode is preferably constructed as a flat surface, which is arranged exactly perpendicularly and symmetrically to the axis of the tube.

In order to produce a completely fiat emissive layer, it is convenient to furnish the front of the cathode with a recess, andto fill out this recess with the eniissive material (for example barium or strontium oxide, or ar'nixture of these oxides), in such fashion that the surface of the emissive layer is situated in exactly the same plane as the surface of the equipotential plane. It is possible in this manner-to employ'consider able quantities of the'highlyemissive material,

and thus to increase appreciably the life of the cathodes. The recess receiving the emissive layer may conveniently be provided with a rectangular form (corresponding "Withthe form of the Now, it is very difiicult, however, to provide a cathode with exactly defined surface form by the use of purely mechanical means.

I have found that it is possible by generation of a suitable electrical field-in the immediate vicinity of the cathode to provide emissive surfaces (referred to in thefollowin'g as virtual cathode surfaces), which behave in exactly thesame manner as if a cathode with an exactly defined geometric surface were employed. The

requisite field according to the present inven tion may be produced in many-di-ffere'ntways, for example by applying a suitable potential between a preferably annular additional electrode ar'-- ranged in the vicinity of the cathode surface and the cathode itself.

Onthe other hand it ispossible' with as a rangemerit according to the invention to compensate' the error entirely when the virtual cathode surface isvaried inaccordance with the particular strength of ray current, which may be done without difficulty by corresponding variation of the field defining the virtual surface.

A cathode ray tube provided with this additional electrode 20 is shown in Fig. 2, the remaining elements of the tube being the same as in Fig. 1.

If there is imparted to the electrode 20 a weak negative bias as compared with the cathode I, a virtual cathode surface results, which possesses a concave form and behaves exactly in the manner of a cathode surface, which is exactly defined geometrically, but from a mechanical standpoint is extremely difficult to produce with the requisite precision. The arrangement has the additional advantage that the structure of the virtual surface is also capable of being varied without diihculty in connection with finished tubes, if necessary during the operation. This may be accomplished by imparting an intensitycontrollin'g variable potential to one of the cylindrical electrodes i3, 20, for instance to the electrode (Wehnelt cylinder) I3, as by means of the elements 2|, 22,24.

The arrangement according to the invention possesses the following essential advantages.

1-. The intensity control and the concentration of the ray are separated, and are, therefore, entirely independent.

2. The working interval is considerably extended; it is possible to obtain a control ranging from the maximum intensity of light capable of being obtained down to absolute darkness.

3. The form of the image point or spot may be made independent of the distribution of the emission on the cathode, and in consequence also of the length of time for which the cathode has been in operation, so that the constancy of the image point or spot is maintained during the entire life of the tube.

4. Since the electrode I4 screens off the cathode against the field of the after-acceleration anode 16, the life ofthe cathode is considerably increased. p

5. The energy requiredfor controlling'the tube is extremely small.

I claim: v I z 1. In a cathode. ray tubecorriprising an indirectly heated equi-potentialcathode, a Wehnelt cylinder surrounding said cathode, a screen on to which a bundle of cathode ray's is to be focused, three apertured electrodes spaced longitudinally between said cathode and said screen and in co-axial alignment with said cathode and Wehnelt cylinder, means to impart to theone of said electrodes adjacent the cathode and Wehm elt cylinder a moderate positive potential; with respect tosaid cathode; means to imparttothe' tured electrodes a high positive potential with respect to said cathode.

2. In a cathode raytube comprising an indi rectly heated equi 'potential cathode to emit electrons, a Wehnelt cylinder surrounding said cathode" to form said electrons into a bundle' of cathode rays, and a screen 0M6 which the bun: dle of said r ys is id be focused; th'r'eeapertured electrodes positioned withtheir apertures in 0:0- axial alignment and spaced one from'another between said Wehneltcylinder and said screen, the apertured electrode nearest said Wehnelt cylinder having a very small aperture, means to impart to the said adjacent electrode a moderate positive potential with respect to said cathode, means to impart to the intermediate electrode of said three electrodes a potential close to the potential of said cathode but lower than said moderate potential, and mean-s to impart to the remaining of said three electrodes a high positive potentialwith respect to said cathode, so that the electron optical system formed by said three electrodes projects an image of said small aperture on to said screen.

3. In a cathode ray tube comprising an indirectly heated equi-potential cathode having an electron emissive area, a Wehnelt cylinder having a negative biaswith respect to said cathode, and a screen adapted to receive a focused bundle of cathode rays from said cathode, an electrode consisting of an annular inner cylinder which is surrounded by said Wehnelt cylinder and itself closely surrounds the electron emissive area of said cathode, three apertured electrodes between said Wehnelt cylinder and said screen with their apertures in axial alignment with the electron emissive area of said cathode and said Wehnelt cylinder, means to impart to said cylinder which closely surrounds the electron emissive area of said cathode a slight negative potential with respect to said cathode, means to apply a potential substantially equal to that of the cathode to the intermediate electrode of said three apertured electrodes, means to apply a moderate potential to the apertured electrode between said intermediate electrode and said Wehnelt cylinder, and means to apply a high positive potential to the remaining electrode of said three apertured electrodes.

4. In a cathode ray tube comprising an indirectly heated cathode, a Wehnelt cylinder surrounding said cathode and a screen on to which a bundle of cathode rays may be focused, a cylindrical electrode surrounding said cathode and positioned between said cathode and said Wehnelt cylinder and three apertured electrodes with their aperturesin co-axial alignment with one another and with said cathode, Wehnelt cylinder and said cylindrical electrode, means to impart to said cylindrical electrode surrounding said cathode a potential negative with respect stantially cathode potential, and means to main- 7 tain the remaining electrode of said three apertured electrodes at a high positive potential with respect to said cathode.

5. In a cathode ray tube comprising an indirectly heated equi-potential cathode, a Wehnelt cylinder surrounding said cathode and a screen on to which a bundle of cathode rays may be focused, a cylindrical electrode co-axial with said cathode and Wehnelt cylinder and between said cathode and said Wehnelt cylinder and three apertured electrodes axially aligned with said cathode, cylindrical electrode and Wehnelt cylinder, and longitudinally spaced between said Wehnelt cylinder and said screen, the aperture of the one of said three electrodes closest to said Wehnelt cylinder being asmall beam limiting aperture, means to impart to said cylindrical electrode and said Wehnelt cylinder negative biases with respect to said cathode so as to focus electrons emitted from said cathode upon said limiting aperture, means to impart to said Wehnelt cylinder a beam intensity controlling variable potential, means to impart to the said one of said three apertured electrodes a moderate positive potential with respect to said cathode, means to impart to the electrode of said three apertured electrodes nearest said screen a high positive potential with respect to said cathode and means to impart to the electrode between the other two of said three electrodes a potential which is approximately cathode potential.

KURT SCI-ILESINGER. 

